cocoa - "type of expression is ambiguous without more context" error in Swift closure -

i'm getting strange type-related error in swift code:

type of expression ambiguous without more context.

this happens if provide full type information.

here's code reproduces it.

i have 2 structures:

struct person{     let name_ : string     let address_ : address }  struct address {     let street_ : string     let city_ : string } 

i create structure contains 2 functions , set address of person:

struct lens<a,b> {     let extract: (a)->b     let create: (b,a) -> } 

when try create instance of lens gets , sets address (in later case returns new person new address), error in first closure.

let lens : lens<person, address> = lens(     extract: {(p:person)->address in         return p.address_}, // here's error     create: {person($0.name_,         address(street_: $1, city_: $0.address_.city_))}) 

not type of parameter of first closure specified in type lens, in closure itself.

what's going on????

while suggests error in extract, in create. $0 , $1 backwards. you're referring $0.name_, $0 of create closure b, address, name_ property of person. think meant:

let lens : lens<person, address> = lens(     extract: { $0.address_ },     create: { person(name_: $1.name_, address_: address(street_: $0.street_, city_: $1.address_.city_)) } ) 

or can redefine lens:

struct lens<a, b> {     let extract: (a) -> b     let create: (a, b) ->   // note, & b swapped } 

and can do:

let lens : lens<person, address> = lens(     extract: { $0.address_ },     create: { person(name_: $0.name_, address_: address(street_: $1.street_, city_: $0.address_.city_)) } ) 

or, perhaps meant:

let lens : lens<person, address> = lens(     extract: { $0.address_ },     create: { person(name_: $0.name_, address_: $1) } ) 

that way, create uses both street , city of supplied address. (it doesn't make sense me use street of address, not city.)